1. Field of the Invention
The present invention relates to a shape memory apparatus which operates for itself or drives loads and the like by utilizing changes of shape of a shape memory element, such as a shape memory alloy (hereinafter referred to as SMA), caused by temperature changes.
2. Description of the Related Art
As an example of the apparatus of this type, there is an apparatus which bends a tubular member, such as an endoscope or catheter, to insert it into an alimentary canal of a living body. In a conventional endoscope, wires are stretched over the full length of its insertion section, and their distal ends are connected to the distal end of a bending portion which is connected to the distal end of the insertion section, while the proximal ends are connected to a knob at a control section which is connected to the proximal end of the insertion section. The wires are loosed and pulled to bend the bending portion by turning the knob. In the case of an endoscope with a relatively long insertion section, e.g., an industrial endoscope, however, it is difficult to obtain a sufficient bend angle by this method because the wires tend to slacken substantially. Moreover, the insertion of the endoscope requires a great deal of skill, and the endoscope sometimes cannot be bent to an optimum angle when it is inserted into an alimentary canal of a complicated configuration, such as the large intestine.
In order to solve these problems, "Study of Servo Actuator Using Shape Memory Alloy" (Shigeo Hirose et al.) was proposed in the 4th Science Lecture Meeting of the Japan Robotological Society (No. 3406, 1986). In this proposed arrangement, the distal end of an insertion section of an endoscope is divided into a plurality of segments, each containing an SMA. The SMA or segment can be bent by heating the SMA with Joule heat to recover a memorized shape of the SMA.
The amount of current supply to the SMA of each segment for generating Joule heat is controlled as follows. First, an operator manually adjusts the amount of current supply to the SMA of the leading segment, while observing an image inside the canal obtained through an image guide fiber. Then, the bend angle of each segment is detected by detecting the resistance of the SMA. In synchronism with the insertion of the endoscope by a distance corresponding to the length of each segment, the detected bend angle is set as a target value of the bend angle for a subsequent segment. Thereafter, the amount of current supply to the segments succeeding the leading segment is controlled in accordance with each target value obtained in this manner. In other words, the bend angle of the leading segment is shifted as the control target value to succeeding segments.
In this conventional example, however, each succeeding segment can bend only to the same angle as the leading segment. When inserting the endoscope of this type into the large intestine or some other portion whose configuration varies as any substance is inserted thereinto, the difference between the respective configurations of the endoscope and the alimentary canal causes the latter to be subjected to an unreasonable force, thus entailing danger.
In the conventional SMA temperature control, the SMA is heated only when it is to be restored to its memorized shape, so that the minimum temperature of the heat cycle applied to the SMA is the environmental temperature (room temperature). If the As' (Austenite start)-point is high, as in the case of a Ti-Ni alloy, the difference between the maximum and minimum temperatures of the heat cycle becomes so great that the minimum temperature is not higher than Ms (Martensite start)-point. Thus, the fatigue life performance of the SMA will be lowered.
These drawbacks may be eliminated by the use of a shape memory alloy apparatus disclosed in Japanese Patent Disclosure No. 61-46475. According to this apparatus, the fatigue life performance of the SMA is improved by heat-biasing the SMA so that the temperature of the SMA is lower than the As'-point and higher than the Ms-point. More specifically, a constant bias current is applied when the recovery of the shape need not be effected. This bias current is set to a value such that the SMA is kept slightly under the As'-point when only the bias current is applied. If a current obtained by superposing a heating current (pulse current) on the bias current is applied, the SMA is heated above the As'-point, thereby recovering the memorized shape.
As described above, this prior art example is based on the assumption that the SMA can be kept slightly under the As'-point if it is supplied with the constant bias current. Actually, however, there is an influence of the environmental temperature (room temperature), so that the SMA cannot always be kept under the As'-point despite the supply of the constant bias current. Thus, the conventional example has no regard for the environmental temperature of the SMA, so that the responsiveness or accuracy may possibly be lowered. If the SMA is at a temperature below the normal working temperature range, for example, its resistance never changes despite a drop of the environmental temperature. Accordingly, the response speed will be lowered if feedback control is based only on the resistance. If the environmental temperature changes, the temperaturedisplacement characteristic of the SMA also changes, so that a desired displacement of the SMA sometimes cannot be obtained.
Disclosed in Japanese Patent Disclosure No. 58-25140, Japanese Utility Model Disclosure Nos. 58-101601, and 61-201018 and Japanese Patent Disclosure No. 59-48710 are endoscopes in which the SMA is contained in its insertion section to effect the bending action of the insertion section. However, these endoscopes are subject to the following drawbacks. In general, a long SMA cannot be shape-memorized. If each segment of the insertion section is independently expected to be bent over a relatively long range by means of the SMA, in order to facilitate insertion into a complicated alimentary canal, such as the large intestine, therefore, a number of shape memory elements must be mechanically connected in series. Thereupon, the individual SMAs elements must bend sustaining the weight of that portion of the insertion section on the distal end side. Thus, in bending all the segments to equal bend angles, the forces needed to bend the segments are greater with distance from the distal end. In other words, the load of the segments on the distal end side and the necessary bending forces therefor are smaller. Even though the SMAs in the individual segments are subjected to the same amount of heating, therefore, the bend angles can not be equalized, so that the insertion section cannot be subjected with uniform bending control.
Disclosed in Japanese Patent Disclosure No. 60-175777, moreover, is an example of a shape memory actuator in which load is driven by utilizing the thermodynamic energy exchanging function of the SMA. In this actuator, a moving part is supported on the underside of a fixed part by means of an SMA coil spring, which is connected to a power source. If the SMA coil spring is energized, it contracts to be restored to its memorized shape, by means of Joule heat produced by conductive heating, thereby raising the moving part. If it is cooled, the SMA coil spring extends, thereby lowering the moving part.
The fixed and moving parts are each provided with a through hole through which the SMA spring is passed, although they are not fixed to the spring. More specifically, the end portions and junctions of the SMA spring are passed through the holes and are bent or knotted.
Meanwhile, in the shape memory actuator, transformation of the SMA coil spring is transmitted to the moving part to operate it. In this case, the force of the SMA coil spring cannot be fully transmitted to the moving part by only passing the spring through the holes in the fixed and moving parts. Thus, the force produced by the SMA cannot be effectively utilized.
The response speed of the SMA is proportional to the periods of time required for heating and cooling the SMA. Therefore, it can be increased by minimizing these periods. In general, the SMA can be heated in a relatively short period of time. On the other hand, there are hardly any effective cooling means. For example, the cooling means for this purpose include ones which use a cooling fluid, such as air, or heat exchanging elements, such as Peltier elements. In the cooling means using the cooling fluid, the fluid must be uniformly circulated through SMA elements, so that each SMA element is contained in a cooling pipe, as described in Japanese Utility Model Disclosure 61-92588. This arrangement is well adapted for the case in which the shape recovering effect of the SMA is utilized for a linear motion. It cannot, however, be effectively used for bending operation. Thus, the elasticity of the cooling pipes constitutes a load on the bending operation. The bending force may be enhanced by thickening the wires of SMA springs or by increasing them in number. If this is done, however, the cooling pipes must be increased in diameter and in number. Thus, this arrangement cannot be easily applied to endoscopes.